1. Field of the Invention
The present invention relates to a lid assembly for an access opening to a subsurface pit for servicing aircraft located beneath an aircraft servicing surface across which aircraft travel while on the ground.
2. Description of the Prior Art
At modern aircraft terminals servicing of aircraft on the ground is frequently performed using prefabricated pits which are installed at aircraft docking, fueling and loading areas. These pits are located beneath the surface of the tarmac across which aircraft travel during docking and departure maneuvers. The pits are typically formed of fiberglass, steel or aluminum and are constructed as enclosures with surrounding walls, and an access lid seated in an opening at the top of the walls. The pits are installed below the surfaces of loading and refueling aprons at aircraft terminals, remote parking locations and at maintenance bases.
The purpose of the pits is to allow ground support functions to be carried out from subsurface enclosures. These ground support functions include the provision of fuel, the provision of electricity to the aircraft while it is in the docking area, the provision of air for cooling the aircraft interior, the provision of pressurized air for starting the aircraft engines, and for other aircraft support activities on the ground. The use of subsurface pits eliminates the need for mobile trucks, carts and other vehicles which are otherwise present in the loading area and which interfere with the arrival and departure of aircraft in the vicinity of a loading gate.
The use of subsurface pits also allows the provision of fuel, power, cooling and pressurized air, and other supplies from a central location. The necessary fluid supplies and electrical power can be generated or stored with a greater efficiency at a central location, as contrasted with mobile generating or supply vehicles.
The pits located below the aircraft terminal area house valves, junction boxes, cooling air terminations and other terminal equipment that is temporarily connected to an aircraft that has been docked. Umbilical pipes and lines, otherwise housed within the pits, are withdrawn from them through hatches therein and are coupled to a docked aircraft to supply it with fuel, air for cooling the aircraft interior, pressurized air for starting the engines, and electrical power.
The pits are constructed with either hinged or totally removable lids that can be moved between open positions allowing access to the pits and closed positions which are flush with the surfaces of the docking, loading or refueling areas across which aircraft travel and beneath which the pits are mounted. Because the pits are located below grade, there is a tendency for water, spilled fuel, dust and debris to fall into the pits through the interstitial cracks surrounding the pit lids within the frames in which the pits are mounted. Since these vertical interstitial gaps represent a point below grade, rainwater and melting snow carries both liquid and solid debris into the gaps surrounding the pit lids. The liquid flows down into the pits carrying some of the debris with it. Also, whenever a pit lid is opened any debris remaining on the shoulder supporting the lid frame is likely to fall into the pit as well.
The entry of dirt, debris and unwanted liquid into the pit enclosure can create problems. Such contaminants accelerate rusting and contribute to jamming of mechanical mechanisms, such as valves and latches. Also, dirt and debris tend to obscure the visibility of dials on pressure and volume gauges, and on dials indicating voltage levels and other readings.
To prevent unwanted contaminants from entering a subsurface pit through the interstitial gaps between the pit lid and the surrounding frame, various sealing systems have been employed. Such conventional sealing systems employ "wiper" seals in which a peripheral seal around a pit lid drags against the surrounding lid mounting frame wall as the lid is seated and unseated relative to the mounting frame. The effect of friction against the mounting frame wall rapidly degrades the integrity of the seal and significantly detracts from the effectiveness of the seal in a relatively short period of time. Thus, conventional pit lid sealing systems have proven unsatisfactory.
Another problem with conventional pit lid sealing systems is that when the seals do degrade they are difficult to replace. Conventional seals are formed of an elastomeric material secured by an adhesive to the edge of the pit lids. When conventional seals become worn and start to leak, they must be pulled away from the lid and the old adhesive must be removed from the edge of the lid before a new replacement seal can be installed. The removal of the old adhesive is a time consuming process, so that worn and deteriorated seals are often not replaced as frequently as they should be.
One prior system which provides a very effective replaceable seal for an aircraft servicing pit lid is described in U.S. Pat. No. 5,404,676. According to this system a pit lid is provided with a flexible, resilient, annular, elastomeric loop which is elastically stretched and removably disposed about the lid to grip it as an encircling jacket. The loop has a plurality of vertically separated, resiliently flexible, radially outwardly extending sealing flaps disposed about its entire perimeter. These flaps are deflected upwardly by contact with the wall of the mounting frame surrounding the access opening so as to establish a liquid-tight seal therewith when the lid is seated in the frame.
While the sealing system of U.S. Pat. No. 5,404,676 is quite effective, certain improvements have been devised. Specifically, in the embodiment disclosed in U.S. Pat. No. 5,404,676 the jacket that extends around and grips the rim of the lid is formed with a generally channel-shaped cross section that includes a pair of upper and lower radially inwardly extending lips. These lips extend over the shoulders of the lid so as to effectuate a grip thereon. While this jacket configuration does provide an effective seal, it is susceptible to damage. For example, in some instances water about the perimeter of the lid that is excluded from entry into the pit will freeze. This prevents removal of the lid until the ice melts or until it is chipped away. In order to free a lid frozen in place in this fashion it is necessary to use some hard implement to strike and chip away at least some of the ice, and then to pry the lid up to break it free from any remaining ice. When this occurs, the elastomeric jacket forming the seal can be damaged.
Some lid assemblies for subsurface pits for servicing aircraft are not hinged to the mounting frame. The access opening to the subsurface pit is exposed by lifting the lid vertically upwardly by means of handgrips cast into the structure of the lid. However, the pit lids are quite heavy and can weigh from fifty pounds to as much as several hundred pounds. Therefore, particularly for larger access openings, the pit lids are hinged to a mounting frame and some type of lift assistance is employed so that the pit lids can be manually raised from a seated, horizontal position. Typically, conventional pit lids of this type are provided with either a counterweight system, such as that described in U.S. Pat. No. 4,467,932, or heavy duty springs to assist in raising the lid. In either case the lid is urged upwardly from the closed position seated in the mounting frame to a fully opened position, preferably through an arc greater than ninety degrees.
Although hinge mechanisms are desirable from the standpoint of raising the lids, they present significant problems to providing water-tight seals. Because the hinge connections of conventional hinged pit lids include hinge-bearing lugs extending radially from the peripheral edge of the lid, it is difficult, if not impossible, to establish a liquid-tight seal along the hinged area of the lid. Moreover, conventional hinge connections to the lid mounting frame often employ bolts that extend through the structure of the frame. These bolt openings form another source of leakage into the pit. As a consequence, rain water, melted snow, dirt, debris, and spilled fuel can all flow into the pit. This leads to corrosion and obstruction of visibility of valves, meters, dials, and the possibility of jammed mechanisms within the pit.
Another problem that arises in conventional sealing systems is that the seal can be damaged when the lid is dropped into position. Since pit lids of this type must be heavy and durable enough to withstand the weight of the tires of a very large aircraft, they must necessarily be heavy and sturdy. Consequently, when the lid descends it frequently does so with a considerable force, even if hinged and counterbalanced by weights or springs. There is thus a significant impact on the elastomeric seal around the edge of the lid. This impact can cause damage to the elastomeric seal.